Every term from the LightningWX Education Center, 492 in all. Each one has a plain-English meaning, the technical definition a meteorologist would use, related terms and the lessons where it shows up. Start anywhere. CAPE is a good one.

Filter The Glossary

0–9

1000-500 Millibar Thickness: The vertical distance between two pressure levels. Thicker layers mean warmer air, so it works like an average temperature for the lower half of the atmosphere.
500 Millibar Level: The height where air pressure is about half of what it is at the ground, usually around 18,000 feet. Forecasters watch it to track the big troughs and ridges that steer storms.
850 Millibar Level: A pressure level usually about 5,000 feet up, above the day-to-day effects of the ground. It shows where warm, moist, or cold air is flowing in the lower atmosphere.

A

ABI: The main camera on the newest GOES weather satellites. It takes pictures of the Earth in 16 different colors of light, from visible to infrared.
Absolute Humidity: How much actual water vapor is packed into a given amount of air, measured as weight per volume.
Absolute Vorticity: The total amount of spin in the air, counting both the spin of the wind itself and the spin it gets from riding on the rotating Earth.
Accretion: When supercooled water droplets freeze onto an ice particle they bump into. It is how graupel, hail, and airplane icing grow.
Adiabatic: A change in air temperature that happens only because the air is squeezed or expanded, with no heat added or taken away. Rising air cools this way; sinking air warms.
Advection: The wind carrying something from one place to another, like warm air, cold air, or moisture.
Advection Fog: Fog that forms when warm, moist air drifts over a colder surface, such as cold ocean water or snow cover, and gets chilled to its dew point.
Advisory: An NWS alert for weather that is a nuisance or somewhat hazardous but is not expected to be dangerous enough for a warning. Be careful, but it is not an emergency.
Aerosol: Tiny bits of dust, salt, smoke, pollen, or pollution floating in the air. Clouds need them to form because water vapor condenses onto them.
Air Mass: A huge body of air, often covering thousands of miles, that has about the same temperature and humidity throughout because it sat over one region for a while.
Air Mass Modification: The way an air mass changes as it moves away from where it formed, picking up or losing heat and moisture from the ground or water beneath it.
Air Pressure: The weight of the air above pressing down on everything. It is about 14.7 pounds on every square inch at sea level and drops as you go up.
Albedo: How much sunlight a surface reflects. Fresh snow reflects most of it; dark ocean water soaks most of it up.
Aliasing: A radar mix-up where winds are faster than the radar can measure cleanly, so they get displayed as blowing the wrong way. It looks like a sudden color flip in the velocity picture.
Altimeter Setting: The pressure number pilots dial into their altimeter so it reads the correct height. Weather reports also give it as the station's pressure.
Altocumulus: Mid-level puffy clouds in rows or patches, often white or gray, sometimes called a "mackerel sky." Each puff looks about thumb-sized at arm's length.
Altocumulus Castellanus: Mid-level clouds with little towers sprouting upward like castle battlements. On a summer morning they hint that thunderstorms may form later.
Altostratus: A gray or bluish sheet of mid-level cloud that covers the sky. The sun looks like it is behind frosted glass, with no halo.
Anemometer: An instrument that measures wind speed, often a set of spinning cups or a small propeller.
Aneroid Barometer: A barometer with no liquid, using a small sealed metal can that squeezes or swells as air pressure changes.
Anomalous Propagation: When layers of warm or cold air bend the radar beam down toward the ground, so the radar shows "echoes" that are really just the ground, buildings, or waves.
Anticyclone: A high pressure system. Air sinks and spreads out, usually bringing fair, calm weather. In the Northern Hemisphere the wind turns clockwise around it.
Anvil: The flat, spreading top of a thunderstorm, shaped like a blacksmith's anvil. It forms when the rising air hits a layer it cannot punch through and spreads sideways.
Anvil Crawler: Long, branching lightning that seems to crawl across the underside of a storm's anvil, sometimes for dozens of miles.
Arcus Cloud: A low, horizontal, often shelf-shaped cloud on the front edge of a storm's cool outflow. It can look menacing but is not itself a tornado.
ASOS: The automated weather stations at most US airports that report temperature, wind, pressure, visibility, and sky conditions every hour or more often.
Atmosphere: The layer of gases wrapped around the Earth, mostly nitrogen and oxygen, held in place by gravity. All our weather happens in it.
Atmospheric River: A long, narrow river of water vapor in the sky, often flowing from the tropics. When one hits mountains it can dump huge amounts of rain or snow.
Aurora: Glowing curtains of green, red, or purple light in the night sky near the poles, caused by particles from the Sun hitting our upper atmosphere.
Auroral Oval: A ring-shaped zone around each magnetic pole where auroras are most common. It grows toward the equator during strong space weather.
Axial Tilt: The Earth leans about 23.5 degrees on its axis. That lean, not our distance from the Sun, is what gives us seasons.
Azimuthal Shear: A radar-based measure of how quickly winds change from blowing toward the radar to blowing away across a small distance. High values mean a storm is spinning.

B

Backbuilding: When new storms keep forming on the upwind side of a storm cluster, so the rain area stays in one place instead of moving on. It is a major cause of flash flooding.
Backdoor Cold Front: A cold front that sneaks in from the east or northeast instead of the usual west or northwest, often bringing a sudden chilly, damp change, especially in New England and the Mid-Atlantic.
Backing: When the wind direction shifts counterclockwise, for example from west to southwest to south.
Ball Lightning: A rare, glowing ball of light, usually the size of a grapefruit, reported during thunderstorms. Scientists are still not sure exactly what it is.
Baroclinic: Describes air where temperature changes a lot across a distance, like near a front. These zones feed the energy of large storms.
Barometer: An instrument that measures air pressure. Falling pressure often signals stormy weather is coming; rising pressure usually means clearing.
Barotropic: Describes air where temperature is about the same everywhere at a given height, like deep in the tropics. Storms here get energy from warm water, not temperature contrasts.
Base Reflectivity: The standard radar picture showing how much energy bounces back from rain, snow, or hail at one beam angle. Brighter colors mean heavier precipitation.
Beam Blockage: When mountains, hills, or tall buildings block part of the radar beam, leaving a blind wedge behind them.
Beam Broadening: The radar beam gets wider the farther it travels, like a flashlight beam. Far from the radar, small features like tornadoes can get blurred out.
Beam Height: How high above the ground the radar beam is at a given distance. Because the Earth curves away, far-off storms are only seen high up.
Beam Width: How wide a radar beam is. The beam spreads out with distance, so far-away storms look blurrier.
Bergeron Process: How most rain and snow outside the tropics starts: in a cloud mixing ice and supercooled water, the ice crystals grow by stealing vapor from the droplets until they get heavy enough to fall.
Black Ice: A thin, clear coating of ice on a road that looks just like wet pavement. It is extremely slippery and hard to see.
Blizzard: A dangerous winter storm with strong winds and falling or blowing snow that makes it nearly impossible to see, lasting for hours.
Bolt From The Blue: A lightning strike that comes out of the side of a storm and hits ground miles away under clear or blue sky. It is why you are not safe just because rain has not reached you.
Bombogenesis: When a low pressure storm strengthens extremely fast, its central pressure dropping sharply in one day. The result is often called a "bomb cyclone."
Boundary Layer: The bottom part of the atmosphere, usually the lowest few thousand feet, where the ground's heating, cooling and friction stir the air.
Bow Echo: A line of storms on radar that bulges out in the middle like an archer's bow. The bulge is where the strongest straight-line winds usually are.
Bright Band: A ring of stronger-looking radar returns at the height where snow is melting into rain. It makes the rain look heavier than it really is.
Brightness Temperature: The temperature a satellite "sees" from the infrared glow of a cloud top or the ground. Very cold readings mean very tall storm tops.
Bulk Shear: How much the wind changes in speed and direction between two heights, such as the ground and 6 kilometers up. More of it helps storms organize and rotate.
Bunkers Motion: A method forecasters use to predict which way a rotating supercell storm will travel, which is often to the right of the average wind.
Buoyancy: The upward push on a bubble of air that is warmer and lighter than the air around it, like a hot-air balloon rising.
BWER: A hole of weak radar returns poking up into a storm, surrounded by heavy precipitation. It marks an updraft so strong that rain and hail have no time to form in it.
Bz: The north-south direction of the magnetic field carried by the solar wind. When it points south, the Earth's magnetic shield opens up and auroras get much stronger.

C

CAPE: A measure of how much energy is available to power rising air in a thunderstorm. Higher numbers mean updrafts can get stronger.
Capping Inversion: A warm layer of air a mile or so up that acts like a lid, keeping storms from forming. If it breaks, storms can erupt explosively.
Carbon Monoxide: A colorless, odorless poisonous gas made by burning fuel. Generators and grills used indoors during storms and power outages can build up deadly levels.
Ceiling: The height of the lowest cloud layer that covers more than half the sky. Pilots need to know it to decide whether they can fly by sight.
Celsius: The temperature scale most of the world uses: water freezes at 0 and boils at 100 at sea level.
Chaos Theory: The idea that tiny differences at the start can grow into huge differences later. It is why weather forecasts lose skill after about one to two weeks.
Chinook: A warm, dry wind that rushes down the east side of the Rocky Mountains. It can melt snow fast and raise temperatures by dozens of degrees in hours.
CIN: The amount of energy holding air back from rising, like a lid on a pot. A little can let heat build; too much keeps storms from forming.
Cirrocumulus: Very high, tiny white puffs of ice cloud in ripples or grains, like fish scales. Each puff looks smaller than your finger at arm's length.
Cirrostratus: A thin, milky veil of high ice cloud covering the sky. It often makes a ring, or halo, around the sun or moon and can signal rain within a day or so.
Cirrus: Thin, wispy, feathery clouds high in the sky, made of ice crystals. Sometimes called "mare's tails."
Clear-Air Turbulence: Bumpy air high up where there are no clouds to warn pilots, usually near the jet stream.
Climate: The typical weather of a place averaged over many years, including its usual ranges and extremes. Weather is what you get today; climate is what you expect.
Climate Normal: The official "average" for a weather statistic, like the normal high temperature on a certain date, based on 30 years of records.
Climate Variability: Natural ups and downs in climate from year to year or decade to decade, such as El Nino years or a string of cold winters.
Cloud Condensation Nuclei: Tiny particles in the air, like salt, dust, or smoke, that water vapor condenses onto to form cloud droplets.
Cloud-To-Ground Lightning: Lightning that strikes the ground. It is the kind that hurts people and starts fires, though most lightning never reaches the ground.
Clutter: Radar echoes that are not weather, like hills, buildings, wind turbines, birds, or bugs.
Cold Air Advection: Wind carrying colder air into an area, making it cooler. It usually follows a cold front.
Cold Air Damming: Cold air trapped against the east side of a mountain range, like the Appalachians, keeping the area chilly and damp while warm air flows over the top.
Cold Front: The leading edge of a colder air mass pushing into warmer air. It often brings a line of storms, a wind shift, and a sharp temperature drop.
Cold Pool: A pool of rain-cooled air that spreads out along the ground beneath thunderstorms. Its edge can lift warm air and start new storms.
Collision-Coalescence: How warm-cloud rain forms: bigger droplets fall faster, bump into smaller ones, and merge until they are heavy enough to fall as raindrops.
Composite Reflectivity: A radar picture that shows the strongest echo found at any height above each spot, so you can see the full intensity of a storm at once.
Condensation: When water vapor turns into liquid water, like droplets on a cold glass or cloud droplets in rising air. It releases heat.
Conditional Instability: Air that is stable if it stays dry but becomes unstable once it is lifted enough to form a cloud. Most thunderstorm environments are like this.
Conduction: Heat moving directly from one thing to another by touch, like the ground warming the air right next to it.
Cone Of Silence: The blind spot directly above a radar, where the beam never points straight up, so storms overhead are only partly seen.
Cone Of Uncertainty: The white cone on hurricane maps showing where the storm's center will probably go. It does not show how big the storm is, and impacts often happen outside it.
Continental Arctic: The coldest air mass, formed over snowy land near the Arctic in winter. It brings bitter cold outbreaks.
Continental Polar: Cold, dry air that forms over land in Canada and northern Asia. It brings clear, chilly weather.
Continental Tropical: Hot, very dry air that forms over deserts, such as the American Southwest and northern Mexico.
Convection: Heat moving by rising and sinking air, like boiling water. In weather, it usually means rising warm air that builds clouds and thunderstorms.
Convection-Allowing Model: A high-detail weather computer model with grid boxes small enough, about 3 km, to draw individual thunderstorms instead of guessing at them.
Convective Outlook: The Storm Prediction Center's daily map showing where severe thunderstorms are possible, rated from Marginal to High risk.
Convergence: When air flows together from different directions and piles up. Since it cannot go into the ground, it rises, which can build clouds.
Coriolis Effect: The way the Earth's spin makes moving air curve, to the right in the Northern Hemisphere and to the left in the Southern. It is why storms spin.
Coronal Mass Ejection: A giant cloud of magnetized gas blasted off the Sun. If it hits Earth one to four days later, it can cause geomagnetic storms and auroras.
Correlation Coefficient: A radar product showing how alike the things in the beam are. Plain rain or snow scores high; mixed precipitation, hail, or tornado debris scores low.
Cumulonimbus: The towering thunderstorm cloud that can reach over 50,000 feet, making lightning, heavy rain, hail, and sometimes tornadoes.
Cumulus: Puffy white "cotton ball" clouds with flat bottoms, common on sunny days. Small ones mean fair weather.
Cumulus Congestus: A tall, growing cumulus cloud that looks like a cauliflower tower. It is the step right before a thunderstorm.
Cyclogenesis: The birth or strengthening of a low pressure storm system.
Cyclone: Any low pressure system with winds spinning around it, counterclockwise in the Northern Hemisphere. The word also names hurricanes in the Indian Ocean.

D

Damage Indicator: A type of building or object, such as a house, barn, or tree, that surveyors use to judge how strong a tornado's winds were.
Damage Survey: A visit by weather service experts after a storm to look at the damage and decide whether it was a tornado or straight-line wind, and how strong.
Data Assimilation: The step where a weather model blends millions of new observations with its last forecast to build the best possible picture of the atmosphere right now.
dBZ: The unit for radar reflectivity. Around 20 is light rain, 40 is moderate to heavy, and above 60 usually means hail.
Dealiasing: The computer step that fixes the radar's wind "mix-ups" so the true wind speeds and directions show correctly.
Debris Ball: A small round blob of strong radar echo at the tip of a hook echo, made by debris lofted by a tornado. It is strong evidence a tornado is on the ground.
Deformation Zone: An area where air is being stretched in one direction and squeezed in another. It often sets up long, heavy bands of snow on the northwest side of winter storms.
Degree Of Damage: A step on a scale describing how badly a particular kind of building or tree was damaged, from minor to totally destroyed.
Dendrite: The classic six-armed, fern-like snowflake. They grow best around 5 degrees Fahrenheit and pile up into fluffy snow.
Dendritic Growth Zone: The layer of the cloud, around 3 to 10 degrees Fahrenheit, where snowflakes grow fastest. Strong lift there means heavy, fluffy snow.
Deposition: When water vapor turns directly into ice without becoming liquid first, like frost forming on a cold night.
Derecho: A long-lived, fast-moving line of storms that causes a wide path of damaging straight-line winds, sometimes hundreds of miles long.
Dew: Water droplets that form on grass and cars on cool, clear nights when surfaces chill below the dew point.
Dew Point: The temperature the air would have to cool to before dew or fog forms. Higher means muggier; above about 65 degrees Fahrenheit feels humid.
Dewpoint Depression: The difference between the temperature and the dew point. A small gap means humid air near saturation; a large gap means dry air.
Differential Phase: A dual-polarization radar measurement that builds up as the beam passes through heavy rain. It helps measure rainfall without being fooled by hail.
Differential Reflectivity: A radar measurement of the shape of what is falling. Big flattened raindrops read positive; round hail or tumbling particles read near zero.
Diffluence: When upper-level winds spread apart like the fingers of a hand. It helps pull air upward from below.
Divergence: When air spreads out from an area. Spreading high up pulls air up from below; spreading near the ground happens where air sinks.
Doppler Effect: The change in pitch you hear when a siren passes. Radar uses the same idea with radio waves to tell if rain is moving toward or away from it.
Doppler Radar: A radar that measures not only where rain is but also how fast it is moving toward or away from the radar, revealing winds and rotation inside storms.
Downburst: A powerful rush of air that slams down out of a thunderstorm and spreads out along the ground, causing damage that can be as bad as a weak tornado.
Downdraft: Air sinking inside or beneath a thunderstorm, usually cooled by rain and evaporation.
Drizzle: Very small, close-together raindrops that seem to float down, usually from low gray stratus clouds.
Dropsonde: A weather instrument dropped by parachute from an airplane, often into hurricanes, to measure temperature, humidity, pressure and wind on the way down.
Dry Adiabatic Lapse Rate: The rate unsaturated air cools as it rises: about 5.5 degrees Fahrenheit per 1000 feet.
Dry Line: A boundary between moist air from the Gulf of Mexico and dry desert air from the west, common in the Plains. Storms often erupt along it in spring.
Dry Slot: A zone of clearing or dry air that wraps into a storm system, visible on satellite as a dark band cutting into the clouds.
Dual-Polarization: Radar technology that sends out both horizontal and vertical radio pulses, letting it tell the shape and type of what it sees, like rain, hail, snow, or debris.
Dust Devil: A small, spinning column of dust on a hot, sunny day. It is not connected to a cloud and is usually harmless, though a few are strong.

E

Echo Tops: The height of the highest part of a storm the radar can detect. Taller tops usually mean stronger storms.
ECMWF: The European weather model, run by a center in England and Italy. It is widely considered one of the most accurate global models.
EF Scale: The scale for rating tornado strength from EF0 (weakest) to EF5 (most violent), based on the damage it causes.
Effective Inflow Layer: The layer of air near the ground that actually feeds a storm's updraft. Forecasters use it to measure shear and spin that matter to the storm.
Effective Shear: A measure of wind shear that adjusts to the depth of the storm, so it works for tall storms and elevated storms alike. About 40 knots or more favors supercells.
El Nino: A periodic warming of the tropical Pacific Ocean that shifts weather worldwide, often bringing wetter winters to the southern US.
Elevation Angle: How far above the horizon the radar dish is pointed. Radars scan at several angles to build a 3D picture of storms.
Energy Budget: A running account of all the energy coming into and going out of the Earth or a place on it. Balance keeps temperatures steady.
Enhanced-V: A V-shaped pattern of very cold cloud tops seen on satellite at the top of a severe storm's anvil. It signals a powerful updraft.
Ensemble: Running a weather model many times with slightly different starting conditions to see the range of possible outcomes. If they agree, confidence is high.
Ensemble Mean: The average of all the forecasts in an ensemble. It is usually more accurate than any single run, though it smooths out extremes.
Ensemble Spread: How much the forecasts in an ensemble disagree. Big spread means low confidence; small spread means high confidence.
ENSO: The see-saw pattern of ocean temperatures and winds in the tropical Pacific that swings between El Nino and La Nina every few years.
Entrainment: When a rising cloud or updraft sucks in surrounding air. If that air is dry, it weakens the cloud.
Environmental Lapse Rate: How fast the temperature actually drops with height at a given place and time, measured by a weather balloon.
Equilibrium Level: The height where a rising air bubble finally becomes as cool as its surroundings and stops being pushed up. Storm anvils spread out near it.
Equinox: The two days each year, around March 20 and September 22, when day and night are about equal everywhere and the Sun is straight over the equator.
Evacuation Zone: An area that local officials may order people to leave when a hurricane threatens, mainly because of flooding from storm surge.
Evaporation: When liquid water turns into water vapor. It takes heat away, which cools the air, as when sweat cools your skin.
Extratropical Cyclone: The typical low pressure storm system of the middle latitudes, with warm and cold fronts, that brings most rain and snow outside the tropics.
Eye: The calm, often clear center of a hurricane, surrounded by the eyewall's fierce winds.
Eyewall: The ring of towering thunderstorms around a hurricane's eye, where the strongest winds and heaviest rain are.
Eyewall Replacement Cycle: When a new ring of storms forms outside a hurricane's eyewall and chokes the old one off. The storm often weakens briefly, then grows larger.

F

Fahrenheit: The temperature scale used in the US: water freezes at 32 and boils at 212 at sea level.
Ferrel Cell: The middle loop of air circulation between about 30 and 60 degrees latitude, where most of the United States lies. It is driven by the loops on either side of it.
Fetch: The distance wind travels over open water. The longer the fetch, the bigger the waves and the heavier the lake-effect snow.
Flanking Line: A row of growing cumulus towers stepping up toward the main part of a thunderstorm, usually on its southwest side.
Flash Flood: A flood that rises very quickly, usually within six hours of heavy rain, a dam failure, or an ice jam. It is the deadliest kind of flood.
Flash Flood Emergency: The most serious flood alert, issued when a severe flash flood is threatening lives and major damage is happening or about to happen.
Flash Flood Warning: An NWS alert meaning flash flooding is happening or about to happen in your area. Move to higher ground immediately.
Flood: When water covers land that is normally dry, from rivers overflowing, heavy rain, snowmelt, or storm surge.
Flood Watch: Conditions could lead to flooding or flash flooding. Be ready to move to higher ground.
Fog: A cloud touching the ground. Officially, it cuts visibility to less than about two-thirds of a mile.
Forward-Flank Downdraft: The main rain-cooled sinking air on the front side of a supercell, where most of its rain and hail fall.
Freezing Drizzle: Drizzle that falls as liquid but freezes on contact with cold surfaces, making roads and sidewalks icy.
Freezing Level: The height where the air temperature drops to freezing (32 degrees Fahrenheit). Above it, clouds hold ice and supercooled water.
Freezing Rain: Rain that falls as liquid but freezes as soon as it hits the cold ground, trees and power lines, coating them in ice.
Friction: The drag the ground puts on wind. It slows the wind near the surface and turns it slightly toward low pressure.
Front: The boundary between two air masses with different temperatures or humidities. Weather often changes when a front passes.
Frontal Lifting: Air being forced upward along a front, where warm and cold air meet. It is a main cause of clouds and rain in weather systems.
Frontogenesis: The process that makes a front stronger by squeezing warm and cold air closer together. It can create narrow bands of heavy snow or rain.
Frost: Ice crystals that form on surfaces on cold, clear nights when the air's water vapor turns directly to ice.
Frostbite: Freezing of skin and the tissue under it from cold, especially fingers, toes, ears, and nose. Strong wind makes it happen much faster.
Fujita Scale: The original tornado damage scale, from F0 to F5, used until 2007 in the US. It was replaced by the Enhanced Fujita Scale.
Funnel Cloud: A cone-shaped cloud hanging from a storm base that is rotating but not touching the ground. If it reaches the ground, it is a tornado.
Fuzzy Logic: A way for computers to make decisions using "sort of" and "probably" instead of strict yes-or-no rules. Radar uses it to guess whether it sees rain, hail, or snow.

G

Gate: One small chunk of the radar beam at a set distance. The radar measures each chunk separately, like pixels along a line.
GEFS: The American ensemble: the GFS weather model run 31 times with small tweaks to show how uncertain a forecast is.
Geomagnetic Storm: A disturbance of Earth's magnetic field caused by a blast from the Sun. It can bring auroras far south and trouble power grids, GPS and radio.
Geopotential Height: The height of a given pressure level above sea level, adjusted for gravity. Weather maps use it to show highs and lows aloft.
Geostationary: An orbit about 22,000 miles up where a satellite circles at exactly the speed Earth spins, so it seems to hover over one spot and can watch weather continuously.
Geostrophic Wind: The theoretical wind that blows along the isobars when pressure pushes and Earth's spin balance each other, a good estimate of winds high above the ground.
GFS: The American global weather model, run by NOAA four times a day and forecasting out to 16 days.
GLM: A lightning camera on GOES satellites that watches for flashes day and night across the Americas and nearby oceans.
GOES: NOAA's weather satellites that sit high over the Americas, giving the satellite pictures seen on TV and in apps.
Graupel: Soft, white, crumbly pellets of snow, sometimes called soft hail, formed when supercooled droplets freeze onto snowflakes.
Gravity Wave: A wave in the air, like ripples on a pond, that forms when stable air is pushed up or down and gravity pulls it back.
Greenhouse Effect: The way gases like water vapor and carbon dioxide trap heat near Earth's surface, keeping the planet about 60 degrees Fahrenheit warmer than it would be otherwise.
Greenhouse Gas: A gas like carbon dioxide, water vapor, or methane that lets sunlight in but traps some of the heat the Earth gives off.
Grid Spacing: The distance between the points where a weather model calculates the weather. Smaller spacing shows finer detail, like individual thunderstorms.
Ground Current: Electricity from a lightning strike that spreads out through the ground. It injures more people than direct strikes do.
Gust: A sudden, brief increase in wind speed.
Gust Front: The leading edge of cool air rushing out from a thunderstorm. When it arrives, the wind shifts and gusts and the temperature drops.
Gustnado: A short-lived swirl of dust or debris along a storm's gust front. It is not attached to the storm's rotation and is usually weaker than a tornado.

H

Haboob: A huge wall of dust pushed ahead by winds rushing out of a thunderstorm, common in deserts like Arizona.
Hadley Cell: A giant loop of air in the tropics. Air rises near the equator, flows toward the poles high up, sinks near 30 degrees latitude, and returns along the ground as the trade winds.
Hail: Balls or lumps of ice that fall from strong thunderstorms. Severe hail is 1 inch (quarter size) or bigger.
Hail Embryo: The small seed, usually a graupel pellet or frozen raindrop, that a hailstone grows around.
Hail Spike: A spike of weak false echo sticking out behind a strong hail core on radar, pointing away from the radar. It is a strong clue of large hail.
Haze: A milky dullness in the air from tiny dust, smoke, or pollution particles, which cuts visibility.
Heat Burst: A sudden rush of hot, dry, gusty wind at night from a dying thunderstorm. Temperatures can jump 10 to 20 degrees in minutes.
Heat Index: How hot it feels when humidity is combined with the air temperature. High humidity makes it feel hotter because sweat does not evaporate well.
Heat Lightning: Lightning from a storm too far away for you to hear its thunder, often seen on summer nights. It is regular lightning, not caused by heat.
Heat Transport: The movement of heat from the warm tropics toward the cold poles by winds and ocean currents. Storms are one of the main ways it happens.
Helicity: A measure of how much the low-level winds can feed spin into a storm's updraft. High values help supercells rotate and can boost tornado chances.
Hodograph: A diagram that plots how wind speed and direction change with height. Its shape tells forecasters what kind of storms to expect.
Hook Echo: A hook shape on radar reflectivity at the back corner of a supercell, formed by rain wrapping around the storm's rotation. Tornadoes often form near the tip.
HRRR: A detailed American short-range weather model updated every hour, good at showing where individual thunderstorms may form in the next day or two.
Humidity: The amount of water vapor in the air. People often mean relative humidity, which compares the moisture to how much the air could hold.
Hurricane: A powerful tropical storm with winds of 74 mph or more that forms over warm oceans in the Atlantic or eastern Pacific.
Hurricane Warning: Hurricane conditions are expected in the area, usually within 36 hours. Complete preparations and follow evacuation orders.
Hurricane Watch: Hurricane conditions are possible in the area, usually within 48 hours. Finish preparations.
Hydrometeor: Any water or ice particle in the air, such as a raindrop, snowflake, hailstone, cloud droplet, or fog droplet.
Hydrometeor Classification: A radar product that uses dual-polarization data to guess what is falling in each spot: rain, snow, hail, graupel, or non-weather targets like birds.
Hydrostatic Balance: The near-perfect balance between gravity pulling air down and pressure pushing it up, which is why air pressure drops with height.
Hygrometer: An instrument that measures how much moisture is in the air.
Hypothermia: A dangerous drop in body temperature from cold, wet, or windy conditions. Shivering, confusion, and slurred speech are warning signs.

I

Ice Nuclei: Rare tiny particles, like certain dust or bits of bacteria, that help water droplets freeze in clouds. Without them, cloud droplets can stay liquid far below freezing.
Ice Storm: A winter storm where freezing rain coats everything in a thick glaze of ice, breaking trees and power lines.
Impact-Based Warning: A warning that tells you not just what is coming but how bad the damage could be, using tags like "considerable" or "destructive."
Inflow: The warm, moist air flowing into a thunderstorm to feed its updraft.
Infrared Imagery: Satellite pictures that measure heat instead of light, so they work at night. Colder, higher cloud tops show up as brighter or more colorful.
Initialization: The starting point of a weather model run, a snapshot of the atmosphere that the model steps forward in time.
Insolation: The amount of sunlight reaching a place. It depends on the season, the time of day, and latitude.
Instability: When the atmosphere lets a bubble of air keep rising once nudged, the way a ball rolls away from a hilltop. It is key for thunderstorms.
Intracloud Lightning: Lightning that stays inside a cloud or jumps between clouds without hitting the ground. It is the most common kind.
Inversion: A layer where the air gets warmer with height instead of cooler. It traps smoke, fog and pollution below it.
Invest: A weather disturbance that forecasters are watching closely because it could become a tropical storm. It gets a number like 91L.
Ionosphere: A high, electrically charged part of the atmosphere, from about 50 to 600 miles up, that reflects radio waves and hosts the aurora.
Isobar: A line on a weather map connecting places with equal air pressure. Tightly packed isobars mean strong wind.
Isodrosotherm: A line on a weather map connecting places with the same dew point.
Isotherm: A line on a weather map connecting places with the same temperature.
ITCZ: A belt of clouds and thunderstorms near the equator where the trade winds from both hemispheres meet.

J

Jet Streak: A faster "wind maximum" inside the jet stream. Its corners are places where air is pulled up from below, which can help storms form.
Jet Stream: A river of very fast wind high in the atmosphere, often over 100 mph, that steers storms and separates cold air from warm air.

K

Katabatic Wind: Cold, heavy air sliding downhill under gravity, like air draining off a glacier or down a mountain valley at night.
KDP: A dual-polarization radar product that is especially good at spotting very heavy rain, even when hail is mixed in.
Kelvin: A temperature scale that starts at absolute zero, the coldest possible temperature. Scientists use it in physics formulas.
Kelvin-Helmholtz Waves: Clouds shaped like rolling ocean waves or breaking curls, formed when a faster layer of air slides over a slower one.
Knot: A speed of one nautical mile per hour, about 1.15 mph. Wind in weather reports is often given in knots.
Kp Index: A 0 to 9 number showing how stirred up Earth's magnetic field is. Higher numbers mean auroras can be seen farther from the poles.

L

La Nina: The cool phase of the Pacific see-saw, when tropical Pacific waters are colder than normal. It often means drier winters in the southern US and more Atlantic hurricanes.
Lake Breeze: A cool wind blowing from a large lake onto land on warm afternoons. Its leading edge can set off thunderstorms.
Lake-Effect Snow: Heavy bands of snow that form when very cold air blows across a warmer, unfrozen lake, picking up moisture and dumping it downwind.
Land Breeze: A gentle night breeze blowing from land out to sea, because land cools faster than water after sunset.
Landfall: When the center of a hurricane or tropical storm crosses the coastline. The worst effects often arrive before the center does.
Landspout: A tornado that forms without a rotating supercell, often from a growing cloud over a boundary. It looks like a thin rope and is usually weak.
Lapse Rate: How quickly temperature drops as you go higher in the atmosphere.
Latent Heat: Hidden heat that water absorbs when it evaporates or melts and releases when it condenses or freezes. It powers thunderstorms and hurricanes.
LCL: The height where rising air cools enough to form a cloud, which usually marks the flat bottom of a cumulus cloud. Low values favor tornadoes.
Left Mover: A storm that splits off and travels to the left of the average wind. In the Northern Hemisphere these usually spin clockwise and tend to produce hail more than tornadoes.
Lenticular Cloud: Smooth, lens- or saucer-shaped clouds that sit still over or downwind of mountains, sometimes mistaken for UFOs.
LFC: The height above which a rising air bubble becomes warmer than its surroundings and keeps rising on its own.
Lifted Index: A simple stability number: negative values mean unstable air that can support thunderstorms; the more negative, the stronger.
Lightning: A giant electrical spark inside a cloud or between a cloud and the ground, heating the air to about 50,000 degrees Fahrenheit and causing thunder.
Lightning Jump: A sudden burst in how often a storm makes lightning, which often comes 10 to 20 minutes before severe weather.
Line Echo Wave Pattern: A line of storms on radar that bends into a wave or S shape. Bows and small spinning areas along the kinks can bring damaging winds or tornadoes.
Local Storm Report: A report the weather service puts out when someone sees or measures severe weather, like hail, damaging wind, a tornado, or flooding.
Longwave Radiation: The invisible heat energy given off by the Earth, clouds, and air. It is how the ground cools at night.
Low-Level Jet: A band of fast wind a few thousand feet above the ground, often at night over the Plains, pumping warm, moist air north and feeding storms.

M

Magnetosphere: The bubble of Earth's magnetic field in space that shields us from most of the solar wind.
Major Hurricane: A hurricane of Category 3 or stronger, with winds of at least 111 mph. These cause most hurricane wind damage.
Mammatus: Rounded, pouch-like bumps hanging from the underside of a cloud, most often beneath a thunderstorm's anvil. They look dramatic but are not themselves dangerous.
Maritime Air Mass: A large body of air that formed over the ocean, so it is humid.
Maritime Polar: Cool, moist air that forms over the northern oceans. It brings clouds, drizzle, and showers to coasts like the Pacific Northwest.
Maritime Tropical: Warm, humid air from tropical oceans like the Gulf of Mexico. It fuels muggy days and many U.S. thunderstorms.
Maximum Estimated Size Of Hail: A radar estimate of the largest hailstones a storm is likely producing.
Maximum Sustained Wind: The strongest steady wind in a tropical cyclone, not counting short gusts. It decides the storm's category.
Melting Level: The height where falling snow and ice start turning to rain.
Meridional Flow: A wavy jet stream pattern where winds swing strongly north and south, bringing big temperature swings.
Mesoanalysis: Detailed, hour-by-hour weather analysis of a small area, used to spot where storms will form and how strong they could get.
Mesocyclone: A rotating column of air, a few miles wide, inside a thunderstorm. It is the engine of a supercell and can produce tornadoes.
Mesohigh: A small bubble of high pressure under or behind a thunderstorm, created by rain-cooled air.
Mesolow: A small area of low pressure, often at the intersection of storm boundaries, where new storms can form.
Mesonet: A dense network of automated weather stations that fills gaps between airport observations.
Mesopause: The coldest boundary in Earth's atmosphere, at the top of the mesosphere.
Mesoscale: The size range of weather features from a few miles to a couple hundred miles, like thunderstorm complexes and sea breezes.
Mesoscale Convective Complex: A huge, roughly round cluster of thunderstorms, often overnight, that can cover an entire state.
Mesoscale Convective System: A large, organized group of thunderstorms acting together as one system, like a squall line or storm cluster.
Mesoscale Convective Vortex: A slowly spinning swirl left behind after a big cluster of storms dies, which can trigger new storms the next day.
Mesoscale Discussion: A short Storm Prediction Center message explaining what forecasters see developing in a specific area over the next few hours.
Mesoscale Sector: A small area that a GOES satellite images every minute so forecasters can watch fast-changing storms.
Mesosphere: The layer of the atmosphere above the stratosphere, where temperatures drop again and most meteors burn up.
METAR: The standard coded weather report from an airport, usually issued every hour.
Meteogram: A chart that shows how the weather at one place is expected to change over time, such as temperature, wind, and rain hour by hour.
Microburst: A small, intense blast of sinking air from a storm that spreads out at the ground with damaging winds, sometimes over 100 mph.
Microscale: The smallest scale of weather, from inches to a mile or so, like eddies behind buildings or dust devils.
Millibar: A unit of air pressure. Normal sea-level pressure is about 1013 millibars.
Mixed Layer: The layer near the ground that gets stirred by daytime heating, so temperature and moisture become fairly even through it.
Mixed-Layer CAPE: A storm-energy estimate based on the average air in the lowest part of the atmosphere, rather than just the surface.
Mixing Ratio: How much water vapor is in the air, measured as grams of water per kilogram of dry air.
Moist Adiabatic Lapse Rate: The rate a saturated, cloudy parcel of air cools as it rises, slower than dry air because condensation releases heat.
Monsoon: A seasonal shift in winds that brings a long wet season, like summer storms in the desert Southwest or India.
Most Unstable CAPE: The storm energy available to the single most buoyant air parcel in the lowest part of the atmosphere, useful for storms rooted above the ground.
Mountain Wave: A wave in the air that forms downwind of mountains when wind blows over them, like ripples downstream of a rock in a river. It can create lens-shaped clouds and severe turbulence.
Multicell: A thunderstorm made of several cells in different stages, with new cells forming as old ones die, so the cluster lasts for hours.

N

NAM: A U.S. weather model that covers North America in more detail than global models, forecasting out to about 3.5 days.
NEXRAD: The United States network of about 160 Doppler weather radars.
Nimbostratus: A thick, dark gray cloud layer that brings steady, long-lasting rain or snow.
NOAA Weather Radio: A network of radio stations run by the weather service that broadcasts forecasts and warnings 24 hours a day. Special receivers can sound an alarm when a warning is issued for your county.
Noctilucent Cloud: Rare, glowing, silvery-blue clouds seen at twilight, far higher than any normal cloud.
Nor'easter: A strong storm that moves up the U.S. East Coast with winds from the northeast, bringing heavy snow or rain, coastal flooding, and beach erosion.
Normalized Rotation: A radar measure of how strong a storm's spin is compared to what is typical, which helps highlight possible tornadoes.
Norwegian Cyclone Model: A classic picture of how a midlatitude storm is born, grows, and dies along a front, developed in Norway about a century ago.
Numerical Weather Prediction: Forecasting the weather by solving the physics equations of the atmosphere on powerful computers.
Nyquist Velocity: The fastest wind speed a Doppler radar can measure without getting confused. Faster winds appear "folded" into the wrong color.

O

Occluded Front: A front that forms when a cold front catches up to a warm front, lifting the warm air off the ground. It usually marks a mature storm.
Omega Block: A stuck weather pattern shaped like the Greek letter omega, which can keep the same weather in place for days.
Orographic Lift: Air forced upward as wind blows over mountains, often making clouds and rain on the upwind side.
Outflow Boundary: The leading edge of cool air rushing out of a thunderstorm. It can trigger new storms and often shows up as a thin line on radar.
Overrunning: When warm, moist air slides up and over a layer of cold air near the ground. It often produces widespread clouds, rain, sleet, or freezing rain.
Overshooting Top: A dome of cloud that bulges above a thunderstorm's flat anvil, showing a very strong updraft.
Ozone Layer: A layer high in the stratosphere with extra ozone that blocks most of the Sun's harmful ultraviolet light.

P

Parameterization: A shortcut weather models use to represent things too small or complex to calculate directly, like individual clouds or turbulence.
Parcel: An imaginary bubble of air that meteorologists follow as it rises or sinks to figure out if storms can form.
Planetary Boundary Layer: The lowest part of the atmosphere, directly affected by the ground: friction, heating, and cooling.
Polar Cell: The loop of air circulation nearest each pole, where cold air sinks at the pole and flows outward along the ground.
Polar Jet: A fast river of wind high up that separates cold polar air from warmer air, steering storms across the middle latitudes.
Polar Vortex: A large area of very cold air and low pressure that circles the North or South Pole, especially in winter.
Polar-Orbiting Satellite: A weather satellite that circles the Earth from pole to pole at a low height, seeing the whole planet in strips about twice a day with fine detail.
Positive Lightning: A rare but very powerful type of lightning that carries a positive charge to the ground. It often strikes far from the storm.
Potential Temperature: The temperature air would have if brought down to sea-level pressure without adding or removing heat. It lets you compare air at different heights fairly.
Precipitable Water: The total amount of water vapor in a column of air, expressed as how deep the water would be if it all rained out.
Precipitation: Any water falling from clouds that reaches the ground, such as rain, snow, sleet, or hail.
Pressure Gradient Force: The push air gets from high pressure toward low pressure. The closer the isobars, the stronger the wind.
Probability Of Hail: A radar estimate of the chance that a storm is producing any hail at all.
Probability Of Precipitation: The chance, as a percent, that measurable rain or snow will fall at a given spot during a forecast period.
Probability Of Severe Hail: A radar estimate of the chance a storm is producing hail at least 3/4 inch across.
Psychrometer: A humidity instrument with two thermometers, one kept wet. Evaporation cools the wet one, and the difference tells how dry the air is.
Pulse Repetition Frequency: How many radar pulses are sent out each second. It controls how far and how fast a radar can measure.
Pulse Storm: A short-lived, single thunderstorm that grows quickly in weak wind shear and can briefly produce hail or a downburst before collapsing.

Q

Quantitative Precipitation Estimate: An estimate of how much rain has already fallen, usually from radar blended with rain gauges.
Quantitative Precipitation Forecast: A forecast of exactly how much rain or melted snow will fall in a given time period.
Quasi-Linear Convective System: A long line of thunderstorms, often with bows and kinks, that can produce damaging straight-line winds and quick, hard-to-warn tornadoes.

R

Radar: A device that sends out radio pulses and listens for the echoes bouncing off rain, snow, and hail to see where precipitation is.
Radar Horizon: The limit beyond which a radar cannot see low parts of storms, because the Earth curves away beneath the beam.
Radial Velocity: The part of the wind moving directly toward or away from the radar.
Radiation: Energy that travels as waves, like sunlight or the heat you feel from a fire, without needing air to carry it.
Radiation Fog: Fog that forms on calm, clear nights as the ground cools and chills the air near it.
Radiative Balance: The balance between the sunlight Earth absorbs and the heat it sends back to space. If it tips, the planet warms or cools.
Radio Blackout: A loss of shortwave radio signals on the sunlit side of Earth caused by a solar flare.
Radiosonde: A small instrument package carried up by a weather balloon that radios back temperature, humidity, pressure, and wind.
Rain Shadow: The dry area on the downwind side of mountains, where air sinks and warms after dropping its moisture.
Rain-Free Base: A dark, flat cloud base under a thunderstorm with no rain falling from it. It marks the storm's main updraft, where a wall cloud or tornado may form.
Range Folding: A radar glitch where a faraway storm shows up at the wrong, closer distance. Radars often mark these areas in purple.
Rapid Intensification: When a tropical storm or hurricane strengthens very quickly.
Rear-Flank Downdraft: A surge of sinking air that wraps around the back of a supercell's rotation. It often helps tighten the spin into a tornado.
Rear-Inflow Jet: A stream of air that rushes into the back of a line of storms and down to the ground, driving damaging winds and bowing the line forward.
Reflectivity: How strongly precipitation bounces the radar signal back. Brighter colors mean heavier rain or hail.
Relative Humidity: How close the air is to being saturated, as a percentage. 100% means the air can't hold any more water vapor at that temperature.
Return Stroke: The bright, powerful flash of lightning that races back up the channel once the downward leader connects with the ground.
Ridge: An elongated area of high pressure, often bringing warm, dry, settled weather.
Right-Front Quadrant: The part of a hurricane with the strongest winds and worst storm surge, to the right of its path in the Northern Hemisphere.
Rime: A white, crusty ice coating that forms when tiny supercooled fog or cloud droplets freeze on contact with objects.
Roll Cloud: A long, horizontal, tube-shaped cloud that is not attached to the storm cloud above and seems to roll along.
Rossby Wave: The giant north-south swings in the jet stream that circle the globe and shape our weather patterns.
Rotational Velocity: A radar measure of how fast air is spinning in a storm, found by averaging the strongest inbound and outbound winds.

S

Safe Room: A room built to strict standards to protect people from tornadoes and hurricane winds, often inside a home.
Saffir-Simpson Hurricane Wind Scale: A 1-to-5 rating of hurricanes based on their strongest sustained winds.
Saturation: The point where air holds as much water vapor as it can at its temperature, so any more cooling makes clouds, fog, or dew.
Saturation Mixing Ratio: The most water vapor air could hold at a given temperature and pressure, in grams per kilogram.
Scud: Low, ragged cloud fragments that often race along under a storm and are sometimes mistaken for tornadoes.
Sea Breeze: A cool wind blowing from the sea onto land on sunny days, often triggering afternoon storms inland.
Sea Surface Temperature: The temperature of the ocean's top layer, which fuels hurricanes and affects coastal weather.
Sea-Level Pressure: Air pressure adjusted as if the weather station were at sea level, so maps can compare places at different elevations.
Sensible Heat: Heat you can feel and measure with a thermometer, as opposed to heat hidden in water vapor.
Severe Thunderstorm: A thunderstorm with hail at least one inch across, winds of 58 mph or more, or a tornado.
Severe Thunderstorm Warning: An alert that a severe thunderstorm is happening or about to happen in your area. Take action now.
Severe Thunderstorm Watch: An alert that conditions are right for severe thunderstorms to develop over the next several hours. Be ready.
Shelf Cloud: A low, wedge-shaped cloud at the front edge of a thunderstorm, marking the gust front and incoming strong winds.
Shortwave Radiation: The sunlight coming from the Sun, including visible light and some ultraviolet and near-infrared.
Shortwave Trough: A small ripple in the upper-level winds that can trigger storms or a burst of snow as it passes.
Side Flash: When lightning hits something tall, like a tree, and jumps sideways to a person standing nearby.
Significant Tornado Parameter: A combined index showing where the ingredients for strong tornadoes overlap.
Simulated Reflectivity: A weather model's guess at what the radar will look like in the future.
Skew-T Log-P Diagram: A chart meteorologists use to plot weather balloon data, showing temperature and moisture at every height and how unstable the air is.
SKYWARN: The National Weather Service's volunteer storm spotter program. Trained spotters report severe weather to help forecasters issue warnings.
Sleet: Small ice pellets that bounce when they hit the ground. They start as snow, melt in a warm layer, then refreeze before landing.
Snow Ratio: How many inches of snow you get from one inch of melted water. Fluffy snow might be 20 to 1; wet, heavy snow might be 6 to 1.
Snow Squall: A short, intense burst of heavy snow and strong wind that drops visibility to near zero, very dangerous for drivers.
Solar Cycle: The Sun's roughly 11-year rhythm of more and fewer sunspots, flares, and storms.
Solar Flare: A sudden, huge burst of energy on the Sun that can disrupt radio communications on Earth within minutes.
Solar Wind: A constant stream of charged particles flowing out from the Sun. When it gusts, it can spark auroras and geomagnetic storms.
Solstice: The longest and shortest days of the year, around June 21 and December 21, when the Sun is farthest north or south.
Sounding: A vertical profile of the atmosphere, showing temperature, moisture, and wind from the ground up, usually from a weather balloon.
Source Region: A large, uniform area, like a big ocean or snowy plain, where air sits long enough to take on its temperature and moisture and becomes an air mass.
Space Weather: Conditions in space, driven by the Sun, that can affect satellites, power grids, radio, GPS, and create auroras.
Spaghetti Plot: A map showing one line from each ensemble forecast. When the lines stay bunched, forecasters are confident; when they scatter, they are not.
Specific Differential Phase: A dual-polarization radar measure that highlights very heavy rain, even when hail is mixed in.
Specific Humidity: The weight of water vapor compared to the total weight of the air, in grams per kilogram.
Spectrum Width: A radar measure of how jumbled the winds are inside each radar pixel, useful for spotting turbulence and boundaries.
Split Flow: A pattern where the jet stream divides into northern and southern branches.
Squall Line: A long line of thunderstorms, often ahead of a cold front, that can bring strong winds and heavy rain as it passes.
Stable Air: Air that resists rising. If pushed up, it sinks back down, so storms have trouble forming.
Station Pressure: The actual air pressure measured at a weather station's own elevation, before any adjustment to sea level.
Stationary Front: A boundary between warm and cold air that is barely moving, which can bring days of cloudy, wet weather.
Steering Winds: The winds higher up that push storms along.
Stepped Leader: The faint, branching channel of charge that zigzags down from a cloud in steps before a lightning strike.
Storm Motion: The direction and speed a storm is moving.
Storm Prediction Center: The national forecast office in Norman, Oklahoma, that forecasts severe storms and tornadoes and issues watches.
Storm Shelter: A reinforced place to go during a tornado, such as a basement shelter, a safe room, or an underground cellar.
Storm Surge: A dangerous rise of seawater pushed ashore by a hurricane's winds, the deadliest part of many hurricanes.
Storm Tide: The total water level during a storm: the normal tide plus the storm surge on top of it.
Storm-Relative Helicity: A measure of how much spin storms can pull in from the low-level winds. Higher values favor rotating storms and tornadoes.
Storm-Relative Velocity: A radar view that subtracts out a storm's movement, making rotation inside the storm easier to see.
Stratiform Precipitation: Steady, widespread rain or snow from layered clouds, as opposed to showery bursts from thunderstorms.
Stratocumulus: Low, lumpy gray or white clouds in patches or rolls, often covering the sky but rarely bringing much rain.
Stratosphere: The layer above where weather happens, where temperatures rise with height because ozone absorbs sunlight. Jets often cruise at its bottom.
Stratus: A flat, gray, low cloud layer that can cover the whole sky, like fog that doesn't touch the ground.
Streamwise Vorticity: Spin in the air aligned with the direction the air is flowing into a storm, like a spiraling football. Updrafts that ingest it tend to rotate.
Subsidence: Slowly sinking air over a large area, which warms and dries as it descends, usually bringing clear skies.
Subtropical Jet: A fast wind current high up near 30 degrees latitude, often bringing moisture and clouds.
Sun Angle: How high the Sun is in the sky. A higher sun heats the ground more because its light is more concentrated.
Sunspot: A dark, cooler patch on the Sun's surface caused by strong magnetic fields. Flares often erupt near them.
Supercell: The most organized and dangerous kind of thunderstorm, with a deep, rotating updraft. Supercells make most large hail and strong tornadoes.
Supercell Composite Parameter: A combined index showing where conditions favor supercell thunderstorms.
Supercooled Water: Liquid water droplets colder than freezing that haven't turned to ice yet. They freeze instantly when they touch something.
Surface Analysis: A weather map of conditions at ground level, with highs, lows, fronts, and isobars drawn in.
Surface Observation: A weather report of conditions at ground level at a specific station.
Synoptic Scale: The size of weather features like big high and low pressure systems and fronts, spanning hundreds to thousands of miles.

T

Temperature: How hot or cold something is. In the air, it reflects how fast the air molecules are moving.
Temperature Inversion: A layer where air gets warmer with height instead of cooler, trapping pollution and capping storms.
Thermometer: An instrument that measures temperature.
Thermosphere: A very high, thin layer of the atmosphere where temperatures can soar above 1000 degrees, and where auroras and the space station are found.
Thickness: The vertical distance between two pressure levels. Bigger thickness means warmer air, and it helps forecasters judge rain versus snow.
Thunder: The loud rumble or crack caused by lightning rapidly heating the air.
Thunderstorm: A storm with lightning and thunder, produced by a cumulonimbus cloud.
Tornadic Vortex Signature: A radar pattern of very tight, strong rotation that suggests a tornado may be forming or on the ground.
Tornado: A violently rotating column of air reaching from a thunderstorm to the ground. The most intense can have winds over 200 mph.
Tornado Debris Signature: A radar sign that a tornado is lofting debris, confirming a damaging tornado is on the ground.
Tornado Emergency: The most urgent tornado alert, used when a violent tornado is confirmed and heading for a populated area. Take cover immediately.
Tornado Warning: An alert that a tornado is happening or radar shows strong rotation near you. Take shelter now.
Tornado Watch: An alert that conditions are favorable for tornadoes in and near the area over the next several hours. Be ready to act.
Tornadogenesis: The process by which a tornado forms. Scientists still cannot reliably predict which storms will produce one.
Training: When storms follow each other over the same place like train cars, causing heavy rain and flash flooding.
Triple Point: The spot where a cold front, warm front, and occluded front meet, or where a dryline meets a front, often a hot spot for severe storms.
Tropical Cyclone: The general name for a spinning, warm-core storm system over tropical waters: depressions, tropical storms, and hurricanes.
Tropical Depression: The weakest stage of a tropical cyclone, with sustained winds of 38 mph or less.
Tropical Storm: A tropical cyclone with winds of 39 to 73 mph. At this point it receives a name.
Tropical Wave: A ripple of low pressure drifting westward through the tropics that can bring showers and sometimes grows into a tropical storm.
Tropopause: The boundary between the troposphere, where weather happens, and the stratosphere above. Storm tops flatten out here.
Troposphere: The lowest layer of the atmosphere, where we live and nearly all weather happens. It gets colder as you go up.
Trough: An elongated area of low pressure, often bringing clouds, rain, and cooler weather.
Turbulence: Chaotic, swirling air motion, the bumps you feel on an airplane.
Typhoon: A hurricane in the western North Pacific Ocean. Same storm, different regional name.

U

Ultraviolet Radiation: Invisible, high-energy sunlight that causes sunburn. Most of it is blocked by the ozone layer.
Unstable Air: Air that keeps rising once it gets a push, like a hot-air balloon, which helps storms grow.
Updraft: A current of rising air inside a cloud or storm. Strong updrafts build tall storms and hold hail aloft.
Upslope Flow: Wind blowing up a gentle slope, like the Great Plains toward the Rockies, cooling the air and making low clouds, fog, or snow.

V

VAD Wind Profile: A chart made by a Doppler radar showing wind speed and direction at different heights above the radar, updated every few minutes.
Valid Time: The exact time a forecast or map is meant to show.
Veering: Winds turning clockwise, either over time or as you go up in height. Veering with height usually means warm air moving in and favors rotating storms.
Velocity Aliasing: When winds are faster than a radar can measure, their colors wrap around to the wrong direction.
Velocity Couplet: A radar pattern with strong winds toward the radar right next to strong winds away from it, showing rotation.
Vertically Integrated Liquid: A radar estimate of how much water and ice a storm is holding. High values suggest heavy rain or hail.
VIL Density: Storm water load divided by storm height, which helps flag storms likely to have large hail.
Virga: Streaks of rain or snow falling from a cloud that evaporate before reaching the ground.
Visible Satellite Imagery: Satellite pictures made from reflected sunlight, like a black-and-white photo of clouds. Only available in daytime.
Volume Coverage Pattern: The scanning routine a radar follows, deciding which tilts it scans and how fast it repeats.
Vorticity: A measure of spin in the air. Positive vorticity (counterclockwise in the Northern Hemisphere) helps storms and low pressure develop.
Vorticity Advection: The transport of spin by the wind. When spin is carried into an area, air tends to rise there, helping clouds and storms form.

W

Wake Low: A small low-pressure area behind a line of storms that can cause sudden strong winds after the rain ends.
Wall Cloud: A lowered, often rotating section of cloud under a thunderstorm's base, where tornadoes can form.
Warm Air Advection: Wind bringing warmer air into an area. It often causes rising air, clouds, and precipitation.
Warm Front: The leading edge of advancing warm air replacing colder air, often bringing steady rain, then warmer, muggier weather.
Warm Nose: A layer of above-freezing air aloft sitting over freezing air near the ground, which causes sleet or freezing rain.
Warm Sector: The warm, humid area between a storm's warm front and cold front, where severe thunderstorms often form.
Warning: A message that dangerous weather is happening now or about to happen. Take action immediately.
Watch: A message that conditions are right for dangerous weather to develop. Be prepared and stay alert.
Water Vapor Imagery: Satellite images showing moisture in the middle and upper atmosphere, even where there are no clouds. Great for seeing jet streams and storm systems.
Waterspout: A tornado or tornado-like spinning column over water.
Weather: What the atmosphere is doing right now or over the next few days: temperature, clouds, wind, rain, and more.
Weather Balloon: A large balloon that carries instruments high into the sky to measure the atmosphere.
Weighting Function: A curve that shows which heights of the atmosphere a satellite channel is mostly "seeing."
Wet-Bulb Temperature: The lowest temperature air can be cooled to by evaporating water into it. It indicates how well sweat can cool you, and helps predict snow.
Wind: Air moving from one place to another, mostly from higher pressure toward lower pressure.
Wind Barb: A symbol on weather maps showing wind direction and speed, with flags and lines on a staff.
Wind Chill: How cold it feels on exposed skin when wind adds to the cold.
Wind Shear: A change in wind speed or direction with height. The right kind of shear helps storms organize and rotate.
Windward: The side of a mountain, island, or object that faces into the wind.
Winter Storm Warning: An alert that heavy snow, sleet, or ice is expected and will make travel dangerous.
Winter Weather Advisory: Snow, sleet, or freezing rain is expected that will be a nuisance and make travel hazardous, but not severe enough for a warning.
WSR-88D: The standard Doppler weather radar used across the U.S., protected inside the familiar white golf-ball dome.

X

X-Band Radar: A short-wavelength radar used for close-range, high-detail scanning, common on mobile storm-research trucks.

Z

Z-R Relationship: The formula that converts radar reflectivity into a rainfall rate.
Zonal Flow: A jet stream pattern running mostly west to east with few big dips, usually bringing fast-moving, less extreme weather.
Zulu Time: The worldwide standard time used in weather and aviation, the same as UTC. For example, 18Z is 1 PM Central Daylight Time.

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